The present invention relates to a single screw extruder with a cylinder which has a melting zone and a homogenizing zone, which adjoins the melting zone, and a screw mounted rotatably in the cylinder, which has at least two process sections, following each other, in the region of the homogenizing zone, the second of which in the conveying direction is constructed as a shear section.
In known single screw extruders it is a constant task of the specialist in the art to increase the throughput of the single screw extruder, at the same time maintaining or even improving the quality of the melt.
A variety of developments exist for this, which relate on the one hand to the embodiment of the rotatably mounted screw in the cylinder, so that in the meantime there have been a variety of screw types of different sorts, which are constructed specifically in the various zones of the extruder. Examples of this are barrier screws or polygon screws with cyclic changes to the channel cross-section. Amongst the polygon screws are ET (Energy Transfer) and wave screws, as are described for example in U.S. Pat. No. 4,405,239 or U.S. Pat. No. 4,015,832 and U.S. Pat. No. 4,173,417.
Developments have also been made in the field of the combination of cylinder- and screw configuration. Thus it is known for example from DE 26 24 589 A1 or DE 199 28 870 C2 in a single screw extruder with a screw constructed as a barrier screw in the region of the melting zone, to provide the cylinder with a groove, whereby the throughput of the single screw extruder can be increased.
In addition, it is known from DE 101 30 759 A1 to embody a plasticizing system in a grooved manner also in the region of a homogenizing zone, which has at least two shear sections and a mixing section. An increase to the throughput can be achieved hereby. However, disadvantages also result through the fact that the maddock or barrier shear section are mounted rotatably here in a grooved cylinder section. Residual solid matter remaining in the raw melt can namely become caught in the grooves and can thus pass over the cross-pieces of the shear section. This effect is also designated as the static drainage effect or ice cube effect. By the passage of the residual solid matter into the melt in adjacent channels of the shear section, the residual solid matter can in fact be well heated on the one hand and at the same time the surrounding melt can be cooled, but on the other hand the danger also exists that non-melted residual solid matter leaves the extruder with the melt and thus impairs the quality of the melt. The shear section therefore can not fulfil its actual task, namely to retain residual solid matter contained in the raw melt and to plasticize this under the effect of moderate shear deformation in the shear gap between cross-pieces of the shear section and the inner wall of the cylinder. In addition, depending on the state of preparation of the raw melt, shear sections can be large pressure consumers.